1. Field of the Invention:
This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device with improved dependence characteristics of contrast ratios and display colors upon viewing angles.
2. Description of the Prior Art:
Liquid crystal display devices have significant advantages in terms of thin size, light weight and low power consumption. They have been widely used in various products such as a watch, an electronic calculator, a word processor, a personal computer and the like. Most liquid crystal display devices employ twisted nematic liquid crystal. Further, simple matrix type liquid crystal display devices, which have been currently used in personal computers generally have a large display plane (about 10 inch diagonally) and large display capacity (e.g., 640.times.480 picture elements). Such a liquid crystal display device has a simple configuration that liquid crystal having a twisted (180.degree. or more) molecular alignment is sandwiched by two spaced glass substrates having transparent striped electrodes formed thereon (ST (super twist) mode). In order to realize a multiplex drive with a relatively large number of scanning lines by use of such a simple configuration, a steepness of electrooptical characteristic must be improved. The steepness represents an electrooptical characteristic of a liquid crystal cell when an applied voltage value of the cell is changed above a threshold voltage. The steepness of electrooptical characteristics can be improved by increasing a total twist angle (referred to as a twist angle) of a molecular alignment in liquid crystal. In practice, a twist angle of a liquid crystal display device in the ST mode must be 180.degree. at a minimum. At this twist angle is much larger compared to the twist angle of about 90.degree. for the TN-LCD, LCDs having such twist angle are referred to as "supertwisted" LCD. However, when a twist angle increases to 180.degree. or more, a display shows undesirable colors because of a birefringence phenomenon. To prevent the undesirable colors, there has been disclosed Japanese Patent Publication No. 63-53528 in which an achromatic display image can be realized by inserting a second liquid crystal cell, which serves to perform optical compensation, between one of polarizers and a first liquid crystal cell (which serves to display), the second cell having a molecular alignment twisted in a direction reverse to that of the first cell. This is based on the principle that light including ordinary ray components and extraordinary ray components is changed into elliptically polarized light by the first liquid crystal cell in which a liquid crystal molecular alignment is twisted. Further, the elliptically polarized light is converted into linearly polarized light by the second liquid crystal cell in such a manner that the ordinary ray components and the extraordinary ray components are replaced with each other. Thus, undesirable coloring, which will be caused by a birefringence phenomenon, can be avoided. As a result, an achromatic display image can be realized. In order to accurately convert elliptically polarized light into linearly polarized light, there must be provided the following conditions:
First, the second liquid crystal cell for optical compensation has a retardation value substantially equal to that of the first liquid crystal cell for display. Second, twist directions of the molecular alignments of both the first and second cells are reversed to each other. Third, their molecular alignments, which are in close contact with each other, must intersect orthogonally.
Besides the above-described technique, there have been disclosed various techniques to prevent a display image from being undesirably colored. For example, retardation films are used in place of a second liquid crystal cell. Specifically, several sheets of retardation films are deposited on a first liquid crystal cell so that the deposited films have substantially the same function as a second liquid crystal cell.
As described above, even in the supertwisted device, a satisfactory a chromatic display image can be obtained when appropriate optical compensation has been provided. Further, with a prescribed combination of color filters, a satisfactory color display image, which is more attractive as a product, can also be obtained. However, in a simple-matrix system, display operations are performed by a multiplexed matrix addressing. Thus, as the number of scanning lines increases in order to increase a display capacity, a difference between a voltage value at which light is cut off and a voltage value at which light is transmitted decreases significantly. As a result, a contrast ratio and a response speed of a liquid crystal display device inevitably deteriorates. Further, in a conventional technique, a display image is reversed or completely disappears, or is undesirably colored depending on viewing directions and angles. These phenomena are essentially disadvantageous to realization of a liquid crystal display device with good quality.
In the case of a liquid crystal display device in an active matrix system, switching elements composed of thin-film transistors and diodes are provided at respective picture elements. In this system, a voltage value at which light is cut off and a voltage value at which light is transmitted can be arbitrarily controlled independently of the number of scanning lines. Therefore, steepness of electrooptical characteristics of liquid crystal need not be significantly high, i.e., a twist angle need not be as large as in the case of a liquid crystal display device in the ST mode.
A liquid crystal cell in a TN (twist nematic) mode, whose molecules are in an orientation of a twist angle of 90.degree., is inferior to a liquid crystal cell in the ST mode in terms of rapidity in electrooptic characteristics. However, a liquid crystal cell in the TN mode utilizes its optical rotatory power as a display principle. Thus, a high-contrast display image can be relatively easily obtained without undesirable coloring. Further, response to voltage in the TN mode is quicker than that in the ST mode. A combination of the active matrix system and the TN mode can realize a liquid crystal display device having a large display capacity, a high-contrast ratio, and quick response to voltage. Moreover, when prescribed color filters are added to the above-described combination, a full-color display image, which is more attractive as a product, can be realized.
However, in this conventional mode, a display image is reversed or completely disappears, or is undesirably colored depending on viewing directions and angles. These phenomena are significantly disadvantageous to realization of a liquid crystal display device of good quality.
To improve such dependence characteristics of a display image upon viewing angles, there has been disclosed Japanese Patent Disclosure No. S62-21423. In this application, a liquid crystal cell and a retardation film, which is a polymer film having optical anisotropy negative in its thickness, are disposed between two spaced polarizers. Further, these has been disclosed Japanese Patent Disclosure No. H3-67219, in which a retardation film is disposed on a liquid crystal cell. This double refraction layer consists of liquid crystal compound (or polymeric liquid crystal) which exhibits a cholesteric liquid crystal phase such that the product of a helical pitch and a refractive index is 400 nm at a maximum. In these references, the consideration has been made only to the case when liquid crystal molecules are aligned perpendicularly to substrates of a liquid crystal cell, but not to the case when liquid crystal molecules are in a twisted alignment, i.e., the case of the TN mode or the ST mode.
The fundamental principle of display operation for the above-described liquid crystal display devices is such that when a voltage is applied to a liquid crystal cell, orientations of liquid crystal molecules therein are changed so as to cause the liquid crystal cell to be optically changed. Thus, if the liquid crystal display device is observed while being inclined to the display surface, the orientations of liquid crystal molecules are observed inaccurately. As a result, a display image is reversed or completely disappears. Particularly in the case of full-color display using prescribed color filters, a display image is significantly deteriorated.